The present invention relates to a method of making a metallic recuperative heat exchanger of the tubular type and, more particularly, to a method of casting a hollow metallic recuperative heat exchanger envelope having fins extending at least from the interior surface of the envelope into the cavity defined by the envelope and, if desired, from the exterior of the envelope.
Because of its resistance to corrosion and erosion, and because of its superior heat transmission capabilities, and also because of its tendency to resist the deposition of particulate matter thereon, cast iron has long been a preferred constituent for the manufacture of heat exchangers particularly for application in transferring heat from a hot flue gas from the combustion of a fossil fuel to preheat air being supplied for the combustion of the fossil fuel. A typical cast recuperator tube, such as shown in U.S. Pat. No. 4,417,615, comprises a hollow elongated metallic envelope having a somewhat rectangular cross-section and defining therein an interior flow cavity through which one of the heat exchange fluids, typically the fluid to be heated, flows while the other heat exchange fluid, particularly the heating fluid, flows over the exterior of the envelope in cross-flow to the fluid passing through the interior of the envelope. It is common to provide fins on both the interior surface and the exterior surface of the heat exchange tube so as to enhance heat transfer between the two fluids.
In U.S. Pat. No. 4,417,615, an integral cast recuperator tube is formed in a two-step casting operation with a sand core being formed with spaces cut therein for the interior fins of the envelope and a split mold wherein spaces are also cut for the exterior fins of the heat exchange tubes. First, the sand core is placed in the lower half of the sand mold and molten metal poured into the cavity therebetween to form the lower half of the heat exchange envelope and then the upper half of the sand mold is placed over the sand core and metal poured into the cavity therebetween to form the upper half of the envelope with the upper and lower halves of the envelope being fused together during the casting of the upper envelope to form a integral one-piece tubular finned heat exchange envelope. Unfortunately, while such casting process produces an acceptable recuperator tubes, the manufacturing process continues to be excessively time consuming and expensive.